I've used it quite a bit, and its been my experience that it doesn't really change the color at all, but it definately does what its supposed to....dull the color some. It seems to kinda depend what color you are spraying it on to. I've noticed that with greens (like POG) it does have the apperance that the color lightens a bit, but nothing real drastic. THe real change is that your stuff will just look duller. At least that has been how my stuff has turned out Hope that helps.
I used it on all my Fett stuff...HOWEVER it was not the final stage of my paintjob....In my opinion it makes the peice look 2-dimensional if it is the last thing done. The layers on Fett are what give it the 3-dimensional look so I use the "Dullcote",but still do my washes and some final weathering after using it. It also helps to shoot some Dullcote on during painting in stages if you're going to mask so as nothing will peel on you when lifting the masking.